An AM demodulator made in accordance with this invention is extremely fast in operation. This is accomplished by splitting the incoming signal into two quadrature components, each then being separately full wave rectified and the outputs of the rectifiers are summed and then subsequently filtered. The output of one full wave rectifier is at double the frequency of the incoming signal and the instantaneous output voltage periodically varies between 0 and 1.41 times the incoming sinewave RMS voltage. The outputs of the two rectifiers are interlaced and their sum is at four times the frequency of the incoming signal with instantaneous voltage periodically varying between 1.41 and 2 times the RMS voltage of the sinewave input signal. The carrier feedthrough ripple is, therefore, much easier to filter being twice the frequency and 42% of the amplitude that would occur with a single full wave rectifier. This means that a low pass filter with less sections and/or higher cutoff frequency can be used for a given amount of carrier feedthrough. It is a well known property of filters that time delay is decreased by reducing the complexity of the filter or by increasing its bandwidth.
There are many types of FM demodulators. One broad category covers all of those that employ amplitude limiting followed by some form of frequency discriminator. The discriminator may be a ratio detector, a phase locked loop, a pulse counting detector, a Foster Sealy discriminator, etc. Amplitude limiting prior to frequency discrimination removes any continuous phase information that might have been available from the incoming tone carrier and all that is left out of the infinitely clipped signal are the positions of the zero crossings. If the FM data modulation is asynchronous, then a limiter/discriminator FM demodulator will cause significant timing error if modulation frequency approaches tone carrier frequency. This is because the frequency of the tone carrier may change in between zero crossings but the FM discriminator will not see this until the next zero crossing. Also, the limiter/discriminator FM demodulator can create an error output due to spiking. The spiking is caused when the signal to noise or the signal to interference ratio becomes low. Under these conditions, there is a very rapid rate of change of phase at the time that the signal and the interference are at phase opposition. The combined amplitude of the two are however low at that instant, at the input of the limiter. If it were not for limiting, the rapid rate of change of phase which creates a high output from the discriminator would be offset by the low amplitude which reduces the output of the discriminator. Hard limiting will however maintain amplitude and therefore very high level spikes will appear at the output of the discriminator. These spikes may create an error in the received data signal.
Another classification of FM demodulators are the linear, continuous phase types. These devices are not preceded by an amplitude limiter (hard clipper). They may be preceded by an amplifier with slow automatic gain control as long as the amplifier passes the input signal waveform without adding non-linear distortion. Various detection means of this type are available, some being phase locked loops, slope detectors, tuned ratio detectors, etc.. The FM demodulator made in accordance with this invention falls into the classification of a linear, continuous phase, ratio detector. However, this discriminator has the advantage that when the incoming signal is at center frequency, no carrier feedthrough ripple appears at the output or even prior to the post detection low pass filter. Most FM demodulators have a high carrier output ripple regardless of the frequency of the carrier within its band of operation. The demodulated signal in fact, may be much lower in amplitude than the carrier ripple, requiring an extensive amount of post detection low pass filtering. Not only does this demodulator have no ripple at center frequency but the ripple increases as frequency is moved away from center, proportionately to the demodulated output voltage. This produces essentially a constant ratio of output voltage to ripple with the ripple being much less than with conventional discriminators. Therefore, the post discriminator low pass filter can have either less sections or a higher cut-off frequency resulting in faster response time.
Since this FM demodulator does not use amplitude limiting and is continuously sensitive to the rate of change of phase of the incoming signal, the output of the demodulator will change simultaneous with a change in the rate of change of phase (frequency) of the incoming signal. The circuit doesn't have to wait for a zero crossing to know what the incoming frequency is. Also, since there is no pre-discriminator limiting, if interference is in the channel and the level of this interference approaches the level of the legitimate signal, then the spiking mentioned above will not occur. The reason for this is that at the point where the signal and interference are at phase opposition and phase is changing most rapidly, the amplitude will also be at minimum. Therefore the output of the discriminator will be attenuated at the point of maximum sensitivity to the interference.